Generally, in a storage apparatus using a physical disk such as a magnetic disk, an optical magnetic disk, an optical disk, and the like, a logical volume is formed on the physical disk in accordance with a request form a host such as a data processing apparatus or the like. In order to back up data in the formed logical volume, generally, the logical volume is copied to a physical disk (copy destination) different from the physical disk (copy source) holding the logical volume.
In such a copy of logical volume, all the data in the logical volume is transmitted to be written from copy source to copy destination sequentially from the top of the data, a predetermined copy processing unit at a time (for example, a data block of 256 Kbytes at a time), and such an operation is called “initial copy”. This initial copy may be set for a plurality of logical volumes and performed at the same time. Hereinafter, conventional copy processing executed in such a case will be described with reference to FIGS. 12 and 13.
FIG. 12 is an illustration schematically illustrating an example in which a plurality of copy sessions conflict with each other in a general storage apparatus. The storage apparatus illustrated in FIG. 12 includes three copy source RAID (Redundant Arrays of Inexpensive Disk) groups #0, #1, #2, and three copy destination RAID groups #10, #11, #12 respectively corresponding to the RAID groups #0 to #2.
Each RAID group is an assembly of a plurality of disks in which RAID 1, RAID 5, or the like is configured, and operates as a virtual disk. The RAID groups #0, #1, #10, #11 are used in a system A for a predetermined use, and the RAID groups #2, #12 are used in a system B for another use different from the predetermined use.
In such a storage apparatus (RAID apparatus), in the example illustrated in FIG. 12, eight copy sessions #0 to #7 are set. Here, the copy session is a management unit for managing a copy source and a copy destination, more specifically, the copy session is a management unit when copying a logical volume as a copy source data area to a logical volume as a copy destination data area. Each logical volume is identified by LUN (Logical Unit Number).
For example, the copy session #0 is a session for copying the copy source logical volume of LUN=01 to the copy destination logical volume of LUN=101. Also, the copy session #1 is a session for copying the copy source logical volume of LUN=02 to the copy destination logical volume of LUN=102. Further, the copy session #2 is a session for copying the copy source logical volume of LUN=03 to the copy destination logical volume of LUN=103. All of the three copy sessions #0 to #2 are copy sessions from the RAID group #0 to the RAID group #10.
In a similar way, the copy session #3 is a session for copying the copy source logical volume of LUN=11 to the copy destination logical volume of LUN=111. The copy session #4 is a session for copying the copy source logical volume of LUN=12 to the copy destination logical volume of LUN=112. All of the two copy sessions #3, #4 are copy sessions from the RAID group #1 to the RAID group #11.
The copy session #5 is a session for copying the copy source logical volume of LUN=21 to the copy destination logical volume of LUN=121. The copy session #6 is a session for copying the copy source logical volume of LUN=22 to the copy destination logical volume of LUN=122. Further, the copy session #7 is a session for copying the copy source logical volume of LUN=23 to the copy destination logical volume of LUN=123. All of the three copy sessions #5 to #7 are copy sessions from the RAID group #2 to the RAID group #12.
Conventionally, the initial copy based on the 8 copy sessions #0 to #7 set as illustrated in FIG. 12 is performed at the same time as described below. FIG. 13 is an illustration illustrating a conventional initial copy procedure in the storage apparatus illustrated in FIG. 12. For example, as illustrated in FIG. 13, data of a predetermined copy processing unit (for example, a data block of 256 Kbytes) is sequentially transmitted to be written from the copy source to the copy destination, in a cyclic sequence of the copy session #0->#1->#2->#3->#4->#5->#6->#7->#0->#1 . . . ->#7->#0->#1-> . . . . At this time, all the data in each copy source logical volume is transmitted to be written sequentially from the top of the data to the copy destination logical volume a predetermined copy processing unit at a time.
In FIG. 13, circled numbers illustrated the sequence of the copy processing, and a state in which the 20th copy processing has been executed is illustrated. In FIG. 13, the data corresponding to the circled numbers 1, 9, 17, . . . in the copy destination logical volume of LUN=101 is copied from a continuous area in the copy source logical volume of LUN=01. In a similar way, the data corresponding to the circled numbers in the other copy destination logical volumes is copied from a continuous area in each corresponding copy source logical volume.
Such copy processing is repeated until all the data in the copy source logical volume of each copy session is copied to the copy destination logical volume. In this way, a plurality of RAID groups can be equally used in the copy processing, so that throughput of the initial copy is expected to improve in the entire apparatus. (refer to Japanese Laid-open Patent Publication No. 2007-213272)
By the way, generally speaking, a physical disk included in each RAID group is a mechanical device constituted by a disk-shaped recording medium called “platter” and a head. The physical disk needs to mechanically move the head to a storing position of access target data on the disk-shaped recording medium, differently from, for example, DRAM (Dynamic Random Access Memory) which stores information by a semiconductor switch operation without a mechanical operation. Such a mechanical movement of the head is an operation taking an extremely long time.
When the copy processing is cyclically executed a predetermined copy processing unit at a time for a plurality of copy sessions as described above, data accesses are performed to distant (dispersed) addresses in a physical disk in the same RAID group. Therefore, every time the copy processing of a predetermined copy processing unit is executed, the head has to be moved to a distant address, so that an amount of the physical movement of the head increases, and only a small performance can be attained, which is far from the maximum performance which the physical disk can originally realize.
Here, the copy source RAID group is used for a normal operation, and the copy destination RAID group is used for a backup. Therefore, generally speaking, more high-speed/high-performance/expensive RAID group is used for the copy source RAID group compared with the copy destination RAID group. To put it the other way around, more low-speed/low-performance/inexpensive RAID group is used for the copy destination RAID group compared with the copy source RAID group. Therefore, in many cases, the decrease in performance due to the increase of head movement amount as described above is significant in the copy destination RAID group.
On the other hand, there are problems as described below when considering the copy source RAID group. When performing copy processing, the copy target data is developed from each copy source RAID group to a cache memory on a predetermined copy processing unit basis, and the data developed in the cache memory on the predetermined processing unit basis is transmitted to the copy destination RAID group. At this time, since, when the data of the predetermined copy processing units is continuously developed from a continuous area, the data which should be developed next can be predicted, a pre-read (prefetch) is performed before a read request is received, and an operation in which the data that may be the copy target is developed in advance is generally performed.
However, when the copy processing is cyclically executed a predetermined copy processing unit at a time for a plurality of copy sessions as described above, data are read from distant (dispersed) addresses in a physical disk in the same RAID group. Therefore, a pre-read (prefetch) from the copy source RAID group to the cache memory cannot be performed at all, so that read performance cannot be improved.